Blender Flame Smoke animation rendering
To display the flame, connect the Volume Info/ Flame socket to the Blackbody intensity socket of your Principled Volume shader



As regards having a sustained flame, without the moment when it "ramps up", I believe the easiest way is to just bake your simulation cache for many frames, then remove the first 66 frames from the animation (Render > Start > 67). It's impossible to skip the baking of the phase where the flames develop, because you can't easily create an already-burning starting condition.

You could in principle move the smoke cache 66 frames forward, but it's hacky (requires moving several files manually) and I wouldn't suggest it.